As a positive photosensitive composition there is disclosed chemically-sensitized positive resist compositions in U.S. Pat. No. 4,491,628 and EP 29,139. The chemically-sensitized positive resist composition is a pattern-forming material which, upon irradiation with radiation such as far infrared rays, produces at the exposed area an acid which is then used as a catalyst in the presence of which a reaction is effected to cause change in the solubility of the irradiated area and unirradiated area in a developing solution, thereby forming a pattern on the board.
The foregoing chemically-sensitized positive resist compositions can be roughly divided into three groups, i.e., ternary positive resist composition comprising an alkali-soluble resin composition, a compound which generates an acid upon irradiation with radiation (photo-acid generator) and a compound having an acid-decomposable group for inhibiting dissolution of the alkali-soluble resin, binary positive resist composition comprising a resin having a group which becomes alkali-soluble upon decomposition by the reaction with an acid and a photo-acid generator and hybrid positive resist composition a resin having a group which becomes alkali-soluble upon decomposition by the reaction with an acid, a low molecular dissolution inhibitive compound having an acid-decomposable group and a photo-acid generator.
JP-A-2-19847 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a resist composition comprising a resin having the phenolic hydroxyl group in a poly(p-hyroxystyrene) entirely or partially protected by a tetrahydropyranyl group.
Similarly, JP-A-4-219757 discloses a resist composition comprising a resin having the phenolic hydroxyl group in a poly(p-hydroxystyrene) substituted by an acetal group in a proportion of from 20% to 70%.
Further, JP-A-5-249682 discloses a photoresist composition comprising a resin similarly protected by an acetal group. Moreover, JP-A-8-123032 discloses a photoresist composition comprising a ternary polymer containing a group substituted by an acetal group.
Further, JP-A-8-253534 discloses a photoresist composition comprising a partly-crosslinked polymer containing a group substituted by an acetal group.
Further, JP-A-8-15864 discloses a positive resist composition comprising as a resin composition a mixture of a polyhydroxystyrene having its hydroxyl group substituted by t-butoxycarbonyloxy group in a proportion of from 10 to 60 mol % and a polyhydroxystyrene having its hydroxyl group substituted by ethoxyethoxy group in a proportion of from 10 to 60 mol %.
Further, JP-A-9-319092 discloses that a resin comprising an acetal group having an oxy bond incorporated therein exerts an effect of lessening standing wave.
However, these prior art photoresist compositions comprising a resin containing an acid-decomposable group leave something to be desired in rectangularity of pattern profile. In particular, these prior art photoresist compositions are disadvantageous in that the corners of the top of profile are rounded.
Further, these prior art photoresist compositions comprising a resin containing an acid-decomposable group are disadvantageous in that they generate marked standing wave and give a pattern profile stained on the side wall thereof. It has thus been desired to overcome these difficulties.